1. Field of the Invention
The present invention relates to a discharge lamp in which a pair of electrodes are provided on the outer surface of a substantially straight glass bulb. Fluophor is applied to the inner surface of the bulb , and a discharge gas such as a rare gas or a rare gas plus metal vapor is charged therewithin.
2. Prior Art
FIG. 2 is a partially cutaway view a fluorescent lamp of an aperture type, disclosed in Japanese Patent KOKAI publication No. 61-185857. The straight glass bulb 1 contains an inert gas and metal vapor such as mercury. The inner wall of the glass bulb 1 is provided with a reflecting film 4 except at an opening 2, and a fluophor 3 is provided on the reflecting film 4. A pair of metal electrodes 5,6 are provided on the outer circumferential surface of the bulb in the proximity of both ends of the bulb. A high frequency generator circuit 7, connected to an ac power source 8, applies a high frequency voltage across the pair of electrodes 5,6. The applied voltage causes a high frequency electromagnetic field to be developed across the electrodes 5,6. This field excites the metal vapor in the glass bulb 1 to emit ultraviolet rays, which in turn cause visible light to be emitted with the aid of fluophor applied on the inner wall of the bulb 1.
A discharge lamp is of simple construction and easy to manufacture. Also the discharge lamp has a long life since it has no filaments therein and is free from the problem where portions near the electrodes become black over time.
The discharge lamp provides more light as the area of electrodes 5 and 6 which contacts with the outer circumferential surface of the glass bulb is increased. However, since the electrodes enclose the glass bulb 1 in the vicinity of both ends thereof, the amount of light emitted is decreased, thereby reducing the effective length in the longitudinal direction of the glass bulb 1. Extending the electrodes in the longitudinal direction increases the area of the electrodes providing more light. On the other hand, extending the electrodes causes a shorter effective length of the bulb 1 that contributes to light emission. The discharge lamps, in which a high frequency is applied across both end portions thereof to cause the discharge, develops a positive column at the middle portion of the bulb. The positive column is highly efficient and uniform in light intensity but the long distance between the electrodes requires a high voltage for initiating discharge.
FIG. 9 is a longitudinal cross-sectional view of a fluorescent lamp disclosed in Japanese Patent KOKAI publication No. 60-12660 and FIG. 10 is an enlarged transverse cross-sectional view thereof. In the figures, the glass bulb 1 is charged with mercury vapor and a rare gas 2 therein and has fluophor 3 applied on the entire inner wall thereof. A pair of electrodes 5 and 6 are disposed on the outer circumferential surface of the glass bulb 1. The high frequency generating circuit 7, connected to a power source by a switch 17 and a plug 13, applies a high frequency voltage across the electrodes 5,6. The applied high frequency voltage causes a discharge 16 in the bulb 1. This discharge excites the mercury atoms to thereby develop ultraviolet rays which in turn cause the fluophor to emit visible light. Such a type of fluorescent lamp suffers from the problem that strong impact by electrons and ions causes deterioration of the fluophor at the portions 4a, 4b on the inner surface of the bulb opposite to the electrodes 5,6, causing the amount of light to decrease with time. Therefore the life-time of the discharge lamp will be shorter in optical information apparatuses, particularly a facsimile apparatus where a change in light output with time can be a problem.